I am quite confused as to when a Catholic can attend an Orthodox divine liturgy. You and other Catholic sources state that a Catholic can't fulfill the Sunday Mass obligation at an Orthodox church. I understand that such is the case if there are Catholic churches in the area. However, if a Catholic is traveling in Greece or Russia on a given Sunday and there is not Catholic church around, I think that in such a case said person can fulfill the Sunday Mass obligation by attending an Orthodox liturgy provided that he or she refrains from receiving Communion.

Answer by Catholic Answers on 8/22/2013:

Frankie--

If a Catholic is traveling in an area in which there is no Catholic church, there is no obligation for the Catholic to attend a eucharistic liturgy on Sundays and holy days. Non-Catholic eucharistic liturgies do not fulfill the Sunday obligation in such a case because no obligation exists when there is not an available Catholic liturgy.

That said, there can be spiritual benefit to attending a valid eucharistic liturgy while traveling, even when that liturgy is offered by a validly-ordained non-Catholic priest. A Catholic may attend an Orthodox liturgy while traveling in a predominantly Orthodox country for the spiritual benefits the liturgy offers. But he does not do so to fulfill the Sunday/holy day obligation.

As for Communion, the Catholic Church does not object to Catholics approaching a validly-ordained non-Catholic minister for valid sacraments when there is no Catholic priest available. Most Orthodox churches though prefer that Catholics not approach Orthodox ministers for the sacraments. That preference should generally be respected. However, if a Catholic has serious reason for needing to receive Communion (e.g., about to board a plane, deployment to a war zone), then the Catholic may approach an Orthodox minister for Communion, and need not mention that he is Catholic unless directly asked.